This application is based on an application No. 11-200369 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to an overfilled light beam optical scanner and an image forming apparatus equipped with the overfilled light beam optical scanner.
(2) Related Art
Conventionally, the laser optical scanner that is used for the image forming apparatus such as the laser printer and the digital copying machine is categorized into the underfilled laser optical scanner and the overfilled laser optical scanner. In the case of the underfilled laser optical scanner, the beam of incident light applied to the deflector has the width smaller than the width of the deflecting facet of the deflector in the direction of scanning. On the other hand, the width of the incident light beam in the case of the overfilled laser optical scanner is larger than the width of the deflecting facet.
High-speed scanning can be realized for the overfilled optical scanner by increasing the number of the reflecting mirrors of the deflector, i.e., a polygon mirror, so that it is not necessary to increase the rotation speed and the size of the polygon mirror unlike the underfilled optical scanner. This is an advantage of the overfilled optical scanner. On the other hand, the amount of light of the beam that is applied to the surface of the photo conductor after deflected by the deflector (referred to xe2x80x9creflected beamxe2x80x9d in this specification) changes as the polygon mirror rotates, i.e., the exposure strength varies as one line of the surface of the photo conductor is exposed.
In order to overcome the problem, solutions have been proposed so as to solve the uneven quantity of light due to the reflected beam change and the luminous flux width change, i.e., so as to expose the surface of the photo conductor with a certain level of exposure strength from the start through the end of the exposure. More specifically, Japanese Patent Laid-Open Publication No. 6-214184 (referred to as xe2x80x9cthe first prior artxe2x80x9d in this specification) discloses how to improve the evenness of light quantity by reducing the first factor. On the other hand, Japanese Patent Laid-Open Publication Nos. 8-160338 (referred to as xe2x80x9cthe second prior artxe2x80x9d in this specification) and 9-211366 (referred to as xe2x80x9cthe third prior artxe2x80x9d in this specification) discloses how to improve the evenness of light quantity by reducing the first and second factors.
According to the first prior art, additional optical element (lens) is disposed between the light source and the deflector to uniformize the intensity distribution of the beam applied to the deflector. According to the second prior art, a filter with irregular transmissivity distribution in the scanning direction is disposed between the light source and the deflector to improve the evenness of the light quantity of beam applied to the photo conductor. According to the third prior art, the principal ray of the beam from the light source is inclined so as to change the proportion of the quantity of the light that is not applied to the deflecting facet (eclipsed part) according to the beam incident angle to the deflecting facet, so that the light quantity of the reflected beam is set to be almost the same.
These proposed solutions, however, are also problematic. As for the first and second prior arts, an optical element such as the lens and the filter needs to be additionally disposed, leading to upsizing and high cost of the optical scanner. As for the third prior art, the beam is unevenly shaped since the eclipsed part is positioned on one side of the beam, and the effect of the light quantity evenness improvement is lessened when a slight error of the position of the light source occurs since high precision is required to incline the principal ray of the beam.
It is accordingly the object of the present invention to provide an overfilled light beam optical scanner and an image forming apparatus provided with the light beam optical scanner that exposes the surface of the photo conductor at a certain level of exposure strength by keeping the light quantity applied to the surface of the photo conductor at a certain level without upsizing and increasing the cost of the light beam optical scanner, changing the beam shape, and precise adjustment of the beam inclination.
The above-mentioned object may be achieved by a light beam optical scanner that includes: a light source that emits a light beam; a deflector having a plurality of deflecting facets that have widths smaller than a width of the light beam, at least two of the deflecting facets being irradiated by the light beam and deflecting the light beam; an optical element that is disposed so that a deflected light beam is applied to the optical element, the deflected light beam being a beam that has been deflected by one of the deflecting facets, the optical element being one of a first optical element through which the deflected light beam passes and a second optical element that reflects the deflected light beam, an incident angle of the deflected light beam applied to an incidence plane of the optical element changing according to an angle of deflection at which the deflecting facet has deflected the light beam; and a thin film with which at least one of an incidence plane of the first optical element, an incidence plane of the second optical element, an emission plane of the first optical element, and the deflecting facet is provided, reflectance and transmissivity of the thin film depending on the incident angle.
In the light beam optical scanner, a thin film that has transmissivity and reflectance depending on the incident angle is disposed on the deflecting facets of the deflector or the plane of incidence or the transmitted light emission plane of the optical element. As a result, the light quantity for exposing the photo conductor, which changes according to the incident angle, is appropriately corrected, so that the surface of the photo conductor is exposed at a certain level of exposure strength.